In general, polyimide has been applied to various fields including insulating substrates for forming circuits and devices due to excellent mechanical properties and heat properties. However, since a charge transfer complex is formed between aromatic rings during polymerization, the polyimide is colored with brown or yellow color, and thus, a transmittance in a visible light region is low, which has difficulty in being applied to display materials.
As a method for preparing the polyimide to be colorless and transparent, a method for suppressing formation of the charge transfer complex in a molecule by using alicyclic diamine or aliphatic diamine as a diamine component has been known. Japanese Patent Laid-Open Publication No. 2002-161136 (Patent Document 1) discloses polyimide obtained by imidization of a polyimide precursor formed with an aromatic acid dianhydride such as pyromellitic dianhydride, etc., and trans-1,4-diaminocyclohexane. Even though the polyimide exhibits high transparency, there is a problem in that mechanical properties are deteriorated.
Further, it has been attempted to use various functional monomers as a method for converting the yellow color of polyimide into colorless transparency. However, there is difficulty in approaching the conversion due to problems in a preparation process, such as a rapid increase in viscosity during polymerization or difficulty in refining, etc., and even though transparency is secured, it is not sufficient to solve a problem that excellent mechanical properties inherent to polyimide are deteriorated.
Meanwhile, in the display material, research into a technology of replacing a cover glass for display with a polymer material has been conducted, and polyimide has received attention as a substitute for the cover glass.
Therefore, it is demanded to develop a technology for polyimide in which superior optical characteristics are exhibited to be applied to various display material fields including a material for replacing the cover glass, excellent inherent mechanical properties are not deteriorated, and in particular high modulus is implemented, thereby further broadening an application range.